1. Field of the Invention
This invention relates to the field of fabrication of transistors and, in particular, in situ recrystallization fabrication wherein the emitters are electrically coupled by the substrate.
2. Background Art
In microwave circuits, it is often desirable to have multistage, common emitter circuits because these circuits are capable of achieving a predictable gain. However, there is often a problem with coupling the emitters of two adjacent stages. If the coupling is done with gold or aluminum wire parasitic inductance associated with the wire will manifest itself as a feedback element at microwave frequencies. This parasitic inductance reduces the speed and the gain of the circuit. Common collector circuits are not suitable because multistage common collector transistor circuits cannot achieve a high gain.
Another way of conventionally coupling stages is the use of capacitive isolation between the stages. This introduces parasitic capacitance which is responsible for speed and gain degradation at microwave frequencies. Attempts to use common collector circuits have not been successful because of the inherently low gain of common collector circuits. The highest speed circuit would be one with a common emitter configuration without any coupling parasitics.
An alternative of building the emitter in the substrate to eliminate the need for external connections between the emitters could not be achieved because the conductivity type of each successive layer in building the transistor must be opposite that of the layer below it. This requires overcompensation at each layer. For example, one could start with a lightly doped substrate and then introduce a P-type dopant to create a P-type layer. The next layer, which must be an N-type layer, must therefore contain enough N-type dopant to first overcompensate for the P-type dopant before it can be further doped to the proper N concentration. This must be repeated for each layer.
The result of this is that the most heavily doped transistor element is on the top, while the most lightly doped and heavily resistive transistor element is on the bottom. The lightly doped resistive bottom transistor element, therefore, has low injection efficiency and is not suitable for use as an emitter. Furthermore, overcompensation in the base causes noise.
A desirable alternative would be to have a layer of undoped monocrystalline semiconductor material for each transistor element, wherein overcompensation for previous doping is not required before doping to the required level. However, when a layer of semiconductor material is deposited upon a substrate during the fabrication process, it is deposited in a polycrystalline structure.
In such polycrystalline structures, there are "grains" or regions of material which are in pure crystalline form. These grains have random orientation with respect to each other. This is to be distinguished from the monolithic, monocrystalline structure formed when rod-pulling is performed using a monocrystalline seed or vapor (liquid) phase epitaxial process. In the polycrystalline structure, the grain boundaries interfere with the electrical properties of the semiconductor material. It is, therefore, desirable to recrystalize the polycrystalline material which has been deposited on a substrate to create monocrystalline material, thereby allowing each transistor element to be doped independently.
Technology for growing single crystalline germanium films in situ is known. For example, in "Single-Crystal Germanium Films by Microzone Melting" by J. Maserjian in Solid-State Electronics, Pergamon Press, 1963, Vol. 6, pp. 477-478, the use of an electron beam to melt a small zone of polycrystalline germanium is taught. J. Douglas, in "The Route to 3-D Chips," High Technology, September, 1983, shows the use of a laser beam, a strip heater, and a focused mercury arc lamp to achieve recystallization. However, the use of laterally proceeding heat sources and small zone techniques such as laser or E-beam heating, create lateral thermal gradients which are difficult to control and adversely affect circuit reliability.
For purposes of considering the patentability of the invention disclosed and claimed herein, a brief patentability search was conducted. The patents identified to be of possible interest in that search were:
______________________________________ Patent Number Inventor(s) ______________________________________ 4,370,670 Nawata, et al. 4,359,754 Hayakawa, et al. 4,345,266 Owyang 4,329,772 Oikawa, et al. 4,255,674 Grenier, et al. 4,190,466 Bhattacharyya, et al. 3,865,648 Castrucci, et al. 3,801,836 Castrucci, et al. 3,619,738 Otsuka ______________________________________
It is, therefore, an object of the present invention to provide recrystallization of polysilicon during fabrication process.
It is a further object of the present invention to provide a processing sequence for bipolar microwave integratable transistors which eliminates the requirement for overcompensation in successive P- and N-type layers.
It is, another object of the present invention to provide an integratable bipolar microwave transistor in which the emitters of a multistage common emitter circuit may be coupled without speed reduction due to the presence of common-lead parasitic inductance and capacitance.